I just finished making an indoor box --> http://slingshotforu...irst-slingshot/
I tried just packing it full of cardboard first, but I quickly discovered that ammo would lodge in the hole and following shots were ricocheting off the buried rounds.
I then added the pvc tubing to add a gap between the front target and the backstop. This gives a nice satisfying sound as the ammo penetrates the cardboard. Using BB's i have a single piece of cardboard in front, using heavier 10mm ammo, I have 2.
I added a piece of carpet, backed by a 3cm piece of foam. Without the foam behind, I found the heavier lead balls were punching holes in the carpet. The rest of the space is taken up by more cardboard cut to shape. When the front target gets too full of holes, I swap it with one of the new pieces in the back.
Ignoring the 10 minutes it took me to run down the store and buy the pvc tube and fittings, the whole thing took about 25-30 mins to make. Size was dictated by what scrap I had availiable.
I intend on making another soon, that has in interior dimension the same as A3 paper for easy targets.